The invention relates to a process for manufacturing a microfilter from a recrystallized aluminum foil using electrolyte etching.
Microfilters are employed as partially permeable membranes in a series of separation processes in which particles having a minimum size of 0.02 to 10 .mu.m in diameter suspended in a gaseous or liquid medium have to be separated out. Using such microfilters for example bacteria in water are held back by the membrane as the water flows through it under a pressure of about 0.1 MPa. The membranes employed up to now are mostly based on polymer foils and hollow polymer foils. These, however, give rise to some difficulties which have serious consequences in certain applications: limited thermal stability (to some extent prohibits sterilization); limited stability in organic solvents, swelling in water, usually no electrical conductivity.
Some help here is a process for manufacturing membranes having a pore diameter 0.2 to 10 .mu.m out of aluminum in which a recrystallized aluminum foil is electrolytically etched. In that case a first etching is employed with a constant anode potential, throughout the duration of the stage and above the pitting potential of the aluminum, and a second etching is employed with a constant anode potential below the pitting potential.
A proposal for narrowing the pores further is made in which an aluminum oxide layer is anodically formed on the etched foils, or a hydrated aluminum oxide layer is formed by treatment in boiling water, as a result of which pore diameters down to 0.002 .mu.m can be achieved.
Aluminum membranes manufactured that way, however, suffer decisive disadvantages:
Potentiostatic etching is, in practice, very difficult to control. There is in particular the danger that the smallest deviation in the effective potential from the optimal value leads to an undesired micro-pitting which excludes the product from use as a membrane foil. Even more decisive is the fact that the membranes manufactured by the above mentioned process are unstable as soon as they are employed for the filtration of aqueous media or non-aqueous media containing the smallest amount of water. This instability is expressed in a rapid drop in trans-membrane flow, which after a few hours is typically only 5 to 10% of the initial value and thus insufficient and uninteresting for microfiltration membranes compared to conventional polymer membrane foils. This drop is observed both with foils etched on one side and foils etched on both sides, with blank foils and with foils having an anodic aluminum oxide barrier layer.